There are a wide variety of wet end additives used for strengthening paper products. For instance, water-soluble polyacrylamides have been used to impart dry strength and temporary wet strength to paper products. Such polymers must have sufficient cationic character to provide an affinity to paper fibers and can be made thermosetting to improve their strength-enhancing character.
One known class of polyacrylamide strengthening agents includes materials which have been modified with glyoxal to make them thermosetting. Coscia et al, U.S. Pat. No. 3,556,932, appears to be the first disclosure of these materials wherein the glyoxalation of vinylamide (e.g., acrylamide) polymers is described as producing a wet strengthening resin. The acrylamide polymer can also include other monomers that confer ionic properties, or act as diluents or spacers. The base polymer is reacted with sufficient glyoxal to form a water-soluble polymer which is thermosetting. The base polymer is reacted with glyoxal under alkaline conditions until a slight increase in viscosity occurs and then is acid quenched. According to the patent, only about half of the added glyoxal reacts and the unreacted glyoxal remains dissolved in the water. The unreacted glyoxal is said to remain in the papermaking whitewater and does not act as a strengthening agent. In one embodiment, the acrylamide polymer and the glyoxal are pre-blended in a dry particulate state and combined shortly before use in warm water.
Glyoxal (CHOCHO) reacts with pendant amide groups on the polyacrylamide backbone (1) according to the following reaction to produce a polyacrylamide having a pendant glyoxalated group (2):

A second reaction involves the so-formed aldehyde moiety on the polymer backbone with another amide group, such as belonging to another macromolecule, and leads to building molecular weight and cross-linking.
Avis, U.S. Pat. No. 3,773,612 teaches wet strength resins comprising a co-polymer of glyoxal, acrylamide, formaldehyde and urea. Acrylamide, formaldehyde and glyoxal are reacted in the presence of isopropyl alcohol, water and N,N dimethylaminoethylmethacrylate to form the methylolated and glyoxalated acrylamide. The reactions are performed at a pH range of from about 8 to about 9 for about 10 to 15 minutes where after the pH is adjusted downward to between about 3.0 and about 5.0. Thereafter, additional acrylamide and urea are added and condensation polymerization reactions between the glyoxalated and methylolated acrylamide and the newly added urea and acrylamide proceed. Ammonium persulfate may then be added to initiate free radical polymerization of the vinyl components of the system. The reaction is stopped when the mixture is diluted with water and hydroquinone.
Ballweber et al. U.S. Pat. No. 4,217,425 teaches a blend of a non-ionic polyacrylamide, polymeric diallyldimethyl ammonium chloride (DADMAC) and glyoxal as a paper strengthening agent. Glyoxal is present to impart crosslinking and the DADMAC functions as a cationic modifier. The blend is prepared under slightly alkaline conditions and held at 40° C. until a desired amount of crosslinking has occurred. An acid kill is then used to decrease the pH from about 7.2 to about 4.0. Thereafter, tetrasodium pyrophosphate is utilized as a buffer.
Bjorkquist, et al, U.S. Pat. No. 4,603,176 describes a glyoxalated acrylamide polymer for imparting temporary wet strength to paper, characterized in that the acrylamide polymer was prepared with a significant amount of a polar, non-nucleophilic monomer that is non-reactive with glyoxal and does not cause the polymer to become water-insoluble. The polar, non-nucleophilic monomer is added to reduce the amount of homo-crosslinking that occurs during use.
Guerro et al, U.S. Pat. No. 4,605,702 also describes a glyoxalated, cationic acrylamide polymer for imparting temporary wet strength to paper, characterized in that the acrylamide polymer has a low molecular weight ranging from about 500 to about 6000 before glyoxalation and the ratio of glyoxal units to acrylamide units ranges from about 0.1–0.5:1.0.
In an attempt to avoid the stability problems that confront glyoxalated polyacrylamide strengthening agents, Dauplaise et al, U.S. Pat. No. 4,954,538 describe microparticles of a cross-linkable, glyoxalated, cationic (meth)acrylamide polymer useful as wet- and dry-strengthening agent prepared using an inverse microemulsion polymerization technique. Dauplaise et al., indicate that commercial glyoxalated acrylamide polymers, supplied as 10% solid solutions, gel within about 8 days at room temperature.
Floyd et al, U.S. Pat. No. 5,147,908 describes a polyvinyl alcohol terpolymer prepared by reacting a polyvinyl alcohol polymer with an aqueous solution of a blocked glyoxal resin and a cationic water-soluble, aldehyde-reactive polymer. The so-produced cationic polyvinyl alcohol additive is highly adsorbent on cellulose pulp and thus suitable for addition to pulp at the wet-end of a paper making process. The glyoxal resin component of this mixture is blocked to prevent it from reacting fully with the other components prior to drying. Inhibiting the reactivity of the glyoxal resin allows a product to be formulated at higher solids and/or lower viscosity than otherwise possible with unblocked glyoxal resin. Suitable blocking components include urea, substituted ureas (such as dimethyl urea), various cyclic ureas such as ethylene urea, substituted ethylene ureas (such as 4,5-dihydroxyethylene urea), propylene urea, substituted propylene ureas (such as 4-hydroxy-5-methyl-propylene urea, or 4-hydroxy-5,5-dimethyl propylene urea), glycols (such as ethylene glycol to make 2,3-dihydroxydioxane, or dipropylene glycol to make an oligomeric condensation product), polyols (i.e. containing at least three hydroxyl groups such as sorbitol or glycerin to make 2,3-dihydroxyl-5-hydroxymethyl dioxane) as well as unalkylated or partially alkylated polymeric glyoxal derived glycols, such as poly(N-1′,2′-dihydroxyethyl-ethylene urea).
U.S. Pat. Nos. 4,508,594; 4,605,718, 5,401,810 all to Jansma et al describe polyacrylamides having acetal and hemi-acetal function purportedly having improved stability.
Due to poor stability, regardless of pH, the molecular weights of current glyoxalated polyacrylamides continue to increase, even at room temperature, until the polymers gel. Thus, the art continues to search for ways for producing glyoxalated polyacrylamide strengthening agents of improved stability.